Formation mechanisms and oxidation behaviour of gold-coloured surface scales in Ni-based single crystal superalloy turbine blades

Surface scale is a recurrent casting defect affecting the quality and reliability of nickel-based single crystal superalloy turbine blades and occurs in a range of colours. In this work, the formation mechanisms of three distinct surface scale types are systematically clarified through high-resolution SEM and TEM analysis, together with observations of unscaled regions and the ceramic mould wall after casting. The surface-scaled regions are shown to correspond to either exposed base alloy surfaces after spallation of an initial alumina, thick double-oxide layers formed under more aggressive oxidation conditions, or a thin nickel oxide film with no evidence of protective alumina. Unscaled regions, by comparison, retain a continuous alumina that prevents further oxidation. These findings demonstrate that surface scales represent a spectrum of oxidation behaviours controlled by oxygen availability during casting, ranging from oxygen-poor to oxygen-rich conditions, which establishes that surface scales are not merely casting defects, but oxidation products. The identification of these distinct oxidation sequences provides a unified framework for interpreting scale formation across different casting conditions. It further provides a quantitative basis for adjusting mould atmosphere and process parameters to minimise scale formation, thereby reducing defects and processing costs. • Surface scale is not merely casting defect but oxidation behaviour. • High resolution observations reveal microstructures of surface scales. • Strong interaction of alloy melt and mould forms protective alumina. • Gold- and black-surface scales are just oxidised surface. • Silver-coloured scales are the exposed surface of base alloys.